1. Field of the Invention
The invention relates generally to multiple function generators for aircraft flight instruments and relates particularly to the maximum operating speed limit pointer driven as multiple and selectable functions of aircraft altitude.
2. Description of the Prior Art
Modern aircraft airspeed indicators often include a maximum operating speed limit pointer which functions to continuously advise the pilot of his maximum allowable airspeed. It is desirable that the indicator provide a plurality of selectable operational airspeed limits to satisfy the different operational requirements of a large number of users of the indicator. Generally, the manufacturers of aircraft instruments are required to provide a large number of customers operating a large number of different aircraft types under a wide variety of operating conditions with suitable instruments. Modern airspeed indicators used by most commercial airlines employ an actual airspeed pointer positioned by a closed-loop servo system in accordance with calibrated airspeed (CAS) received from the aircraft's air data computer, the pointer being read against an airspeed dial calibration. Such indicators normally include the warning pointer which indicates to the pilot the maximum airspeed the is permitted to fly under various flight conditions and aircraft configurations. Such limits are referred to as V.sub.MO or M.sub.MO (speed, maximum operating) in accordance with whether airspeed or mach number is the significant parameter. This maximum operating speed is typically a predetermined mach number (M) and/or equivalent airspeed (EAS). At higher altitudes, the limit is usually based on mach number while at lower altitudes, the limit is often based on equivalent airpseed. Both of these limits are functions of altitude so that the maximum operating speed pointer must be driven or positioned as a function thereof. However, each airframe manufacturer, airline operator or national regulatory agency may establish for the type of aircraft in question varying limit requirements whereby it is necessary that the instrument maufacturer be able to satisfy such requirements by relatively simple instrument modifications.
It is known that the mach number limit and the equivalent airspeed limit are non-linear functions of calibrated airspeed and altitude. While some prior art airspeed indicators provide multiple function mach number and equivalent airspeed limits, the mechanisms for providing such limits were not precision functions but were typically approximations of the actual non-linear functions, such approximations being achieved using traditional mechanical means. Because of the inherent functional accuracy limitations, flexibility in operating the aircraft at its optimum airspeed without exceeding safety margins to maximize operational efficiency was hindered.
In most prior art devices, the above functions were provided by complex electronic function generation apparatus or the previously mentioned mechanically selected function generation devices involving either complex mechanisms with limited achievable accuracy or simple mechanisms with limited capability. Additionally, such instruments were not readily adaptable in accordance with the various requirements described above. In some prior art instruments, the maximum allowable airspeed function was implemented by driving the needle through a cam contoured in accordance with one of the desired functions. A second function was implemented by interposing a mechanism to engage the cam follower whereby the cam follower became disengaged from the cam at a particular point on the curve. Thus the second function was constrained to be identical to the first function in the area where the cam follower remained engaged with the cam and to provide a constant airspeed limit value after disengagement. Thus with this arrangement, flexibility and accuracy could not be achieved since the second function was constrained in accordance with the first function and in accordance with the constant value permitted by the mechanism. Additionally, the second function was constrained to provide its constant value at a lower airspeed than the upper airspeed boundary of the first function. In order to achieve flexibility and accuracy with this arrangement, one cam for each function would be required with an exceedingly complicated mechanism for moving the cam follower from one cam to another in order to change functions. In the limited space available in an aircraft flight instrument, this arrangement would be prohibitively bulky ad unreliable. Additionally, since a greater number of mechanical components are involved, the achievable accuracy would be considerably less.